1. Field of the Invention
The present invention relates to a liquid crystal display device used as, for example, a display unit of an electronic apparatus.
2. Description of the Related Art
In recent years, liquid crystal display devices have come to be used for TV receivers, monitor devices of personal computers, etc. In these purposes, liquid crystal display devices are required to have a good viewing angle characteristic that the display screen is viewable from all directions. FIG. 20 is a graph showing transmittance vs. application voltage characteristics (T-V characteristics) of a VA (vertically aligned)-mode liquid crystal display device. The horizontal axis represents the voltage (V) applied to the liquid crystal layer and the vertical axis represents the light transmittance. Line A is a T-V characteristic obtained in the direction perpendicular to the display screen (hereinafter referred to as “front direction”), and line B is a T-V characteristic obtained in a direction having an azimuth angle of 90° and a polar angle of 60° with respect to the display screen (hereinafter referred to as “slant direction”). The azimuth angle is measured counterclockwise from the rightward direction of the display screen and the polar angle is measured from the perpendicular to the display screen at the center.
As shown in FIG. 20, distortion exists in transmittance (luminance) variations in a region enclosed by circle C and its neighborhood. For example, whereas the transmittance in the slant direction is higher than that in the front direction at a relatively low gradation level (application voltage: about 2.5 V), the transmittance in the slant direction is lower than that in the front direction at a relatively high gradation level (application voltage: about 4.5 V). As a result, when the display screen is viewed from the slant direction, luminance differences in an effective drive voltage range are small. This phenomenon is most remarkable in color variations.
FIGS. 21A and 21B show a difference in visual recognition between images displayed on the display screen. FIG. 21A shows an image as viewed from the front direction and FIG. 21B shows an image as viewed from the slant direction. As seen from FIGS. 21A and 21B, the image looks more whitish when viewed from the slant direction than when viewed from the front direction.
FIGS. 22A to 22C are gradation histograms of the three primary colors of red (R), green (G), and blue (B) of a reddish image, respectively. FIG. 22A is the gradation histogram of R, FIG. 22B is the gradation histogram of G, and FIG. 22C is the gradation histogram of B. The horizontal axes of FIGS. 22A to 22C represent the gradation (256 gradations (0 to 255)) and their vertical axes represent the percentage of presence. As seen from FIGS. 22A to 22C, in this image, relatively high gradation levels of R exist at high percentages and relatively low gradation levels of G and B exist at high percentages. If this kind of image is displayed on the display screen of a VA-mode liquid crystal display device and viewed from the slant direction, R (high gradation levels) looks relatively darker and G and B (low gradation levels) look relatively brighter. Since the luminance differences between the three primary colors decrease, the image looks whitish as a whole.
The above phenomenon occurs in a similar manner also in liquid crystal display devices of the TN (twisted nematic) mode, which is an older drive mode. JP-A-2-12 (Reference 1), U.S. Pat. No. 4,840,460 (Reference 2), and Japanese Patent No. 3,076,938 (Reference 3) disclose techniques for suppressing the above phenomenon in TN-mode liquid crystal display devices. FIG. 23 shows the configuration of one pixel of a basic liquid crystal display device according to these prior art references. FIG. 24 is a sectional view of the liquid crystal display device taken along line X-X in FIG. 23. FIG. 25 shows an equivalent circuit of one pixel of the liquid crystal display device. As shown in FIGS. 23 to 25, the liquid crystal display device has a thin-film transistor (TFT) substrate 102, a counter substrate 104, and a liquid crystal layer 106 which is sealed between the two substrates 102 and 104.
The TFT substrate 102 has plural gate bus lines 112 formed on a glass substrate 110 and plural drain bus lines 114 formed so as to cross the gate bus lines 112 with an insulating film 130 interposed in between. A TFT 120 which is formed as a switching element for each pixel is disposed close to the crossing point of each set of a gate bus line 112 and a drain bus line 114. Part of the gate bus line 112 associated with the TFT 120 functions as a gate electrode of the TFT 120, and a drain electrode 121 of the TFT 120 is electrically connected to the associated drain bus line 114. A storage capacitance bus line 118 is formed so as to traverse a pixel region defined by the gate bus lines 112 and the drain bus lines 114 and to extend parallel with the gate bus lines 112. A storage capacitance electrode 119 which is provided for each pixel is formed above the storage capacitance bus line 118 with the insulating film 130 interposed in between. The storage capacitance electrode 119 is electrically connected to a source electrode 122 of the TFT 120 via a control capacitance electrode 125. A storage capacitor Cs is formed by the storage capacitance bus line 118, the storage capacitance electrode 119, and that part of the insulating film 130 which is interposed between them.
The pixel region which is defined by the gate bus lines 112 and the drain bus lines 114 is divided into sub-pixels A and B. A pixel electrode 116 is formed in the sub-pixel A, and a pixel electrode 117 which is separated from the pixel electrode 116 is formed in the sub-pixel B. The pixel electrode 116 is electrically connected to the storage capacitance electrode 119 and the source electrode 122 of the TFT 120 via a contact hole 124. On the other hand, the pixel electrode 117 is in an electrically floating state. The pixel electrode 117 has a region that coextends with part of the control capacitance electrode 125 with a protective film 132 interposed in between. In this region, a control capacitor Cc is formed by the pixel electrode 117, the control capacitance electrode 125, and that part of the protective film 132 which is interposed between them. The pixel electrode 117 is connected indirectly to the source electrode 122 via the control capacitor Cc (capacitive coupling).
The counter substrate 104 has a color filter (CF) resin layer 140 formed on a glass substrate 111 and a common electrode 142 formed on the CF resin layer 140. A liquid crystal capacitor Clc1 is formed in the sub-pixel A by the pixel electrode 116, the common electrode 142, and that part of the liquid crystal layer 106 which is interposed between the electrodes 116 and 142, and a liquid crystal capacitor Clc2 is formed in the sub-pixel B by the pixel electrode 117, the common electrode 142 and that part of the liquid crystal layer 106 which is interposed between the electrodes 117 and 142. Alignment films 136 and 137 are formed at the interfaces between the TFT substrate 102 and the liquid crystal layer 106 and between the counter substrate 104 and the liquid crystal layer 106, respectively.
Now assume that the TFT 120 has been turned on, whereby a voltage is applied to the pixel electrode 116 in the sub-pixel A, that is, a voltage Vpx1 develops across that part of the liquid crystal layer 106 which is located in the sub-pixel A. Since the voltage Vpx1 is divided according to the capacitance ratio of the liquid crystal capacitor Clc2 and the control capacitor Cc, a voltage that is applied to the pixel electrode 117 in the sub-pixel B is different from the voltage applied to the pixel electrode 116. A voltage Vpx2 that develops across that part of the liquid crystal layer 106 which is located in the sub-pixel B is given byVpx2={Cc/(Clc2+Cc)}×Vpx1.It is ideal that the voltage ratio Vpx2/Vpx1 (=Cc/(Clc2+Cc)), which is a design item that should be set according to intended display characteristics of an actual liquid crystal display device, be set approximately at 0.6 to 0.8.
Where as described above each pixel has the sub-pixels A and B in which different voltages develop across the corresponding portions of the liquid crystal layer 106, the distortion in the T-V characteristic as shown in FIG. 20 is shared between the sub-pixels A and B. Therefore, the phenomenon that an image looks whitish when viewed from a slant direction is suppressed and the viewing angle characteristic is improved. The above technique will be referred to below as “capacitive coupling HT (halftone/gray scale) technique.”
Although in References 1 to 3 the above technique is discussed for TN-mode liquid crystal display devices, its effect is enhanced if the above technique is applied to a liquid crystal display device of the VA mode which has become the mainstream mode in recent years in place of the TN mode.
FIGS. 26A to 26D illustrate a burn-in phenomenon occurring in a conventional liquid crystal display device that employs the capacitive coupling HT technique. FIG. 26A shows a black-and-white checkered pattern that was displayed on the screen in a burn-in test. In the burn-in test, a halftone image (32/64 gradations) of the same gradation level was displayed over the entire screen immediately after the checkered pattern of FIG. 26A had been displayed continuously for a prescribed time (e.g., 48 hours) and it was checked whether a checkered pattern was seen. If a checkered pattern was seen, the luminance of the screen was measured along one direction of the checkered pattern and a burn-in factor was calculated. The burn-in factor is defined as b/a where a is luminance of low-luminance regions of a visually recognized checkered pattern and a+b (>a) is luminance of high-luminance regions.
FIG. 26B shows a state of the screen on which a halftone image was displayed in a liquid crystal display device that did not employ the capacitive coupling HT technique. FIG. 26C shows a state of the screen on which a halftone image was displayed in a conventional liquid crystal display device that employed the capacitive coupling HT technique. As shown in FIG. 26B, a checkered pattern was hardly seen when displaying the halftone image in the liquid crystal display device that did not employ the capacitive coupling HT technique. When the luminance was measured along line Y-Y′ in FIG. 26B, a luminance distribution represented by line c in FIG. 26D was obtained. The burn-in factor was as small as 0% to 5%. In contrast, a checkered pattern as shown in FIG. 26C was seen in the liquid crystal display device that employed the capacitive coupling HT technique. When the luminance was measured along line Y-Y′ in FIG. 26C, a luminance distribution represented by line d in FIG. 26D was obtained. The burn-in factor was 10% or more. As is understood from this test, whereas almost no burn-in occurs in a liquid crystal display device that does not employ the capacitive coupling HT technique, a relatively high degree of burn-in occurs in a liquid crystal display device that employs the capacitive coupling HT technique.
A burn-in distribution in each pixel and other items of liquid crystal display devices where a burn-in phenomenon occurred were evaluated and an analysis was done. And it was found that the burn-in phenomenon occurs in the sub-pixels B where the pixel electrode 117 is formed which is in an electrically floating state. The pixel electrode 117 is connected to the control capacitance electrode 125 via a silicon nitride film (SiN film) or the like having a very high electrical resistance, and is connected to the common electrode 142 via the liquid crystal layer 106 also having a very high electrical resistance. Therefore, the charge accumulated in the pixel electrode 117 is not released easily. On the other hand, a prescribed voltage is written frame-by-frame to the pixel electrode 116 of the sub-pixel A which is electrically connected to the source electrode 122 of the TFT 120, and the pixel electrode 116 is connected to the drain bus line 114 via the operation semiconductor layer of the TFT 120 which is much lower in electrical resistance than the SiN film and the liquid crystal layer 106. Therefore, there does not occur an event that the charge accumulated in the pixel electrode 116 is not released.
As described above, conventional liquid crystal display devices that employ the capacitive coupling HT technique have a problem that they cannot provide superior display characteristics because of occurrence of the burn-in phenomenon though their viewing angle characteristic is improved.
JP-A-2004-78157 and JP-A-2003-255303 are other prior art references relating to the invention.